Rice can be prepared and frozen ahead of time and re-heated in the microwave for use in your bento.
Just cook the rice normally, then when it is still warm (important!), measure out portions of rice and wrap it in strong plastic wrap or pack the rice into ziplock bags. When the packs of rice have cooled down, pop them into the freezer.
To re-heat, take out a packet of rice, and warm it up in the microwave. Please ensure your plastic bag/wrap is microwave-friendly. (Ziploc brand bags recommend you open up the bag to create a vent so steam can escape when you microwave it)
2. Freezing Onigiri (from Just Bento)
Just as you can freeze rice (see above tip), you can freeze onigiri (rice balls). After you have shaped the onigiri, wrap it in strong plastic wrap when still warm. Let it cool and then freeze it.
Microwave to re-heat.
3. Be a squirrel and build-up a Bento Stash
It's not necessary to horde food up for a whole winter of bento-making, but it saves time if you build-up a little Bento Stash in your freezer and/or fridge.Freezer stash: Make a large batch of meatballs and freeze them wrapped in little packs of 4 or 8 (depending on how many you think you will defrost at a time).
Keep some frozen vegetables such as sugar peas, and edamame so you have vegetables on hand at any time.
You can also freeze berries such as raspberries, blueberries and strawberries (hull them first). Blueberries hold up the best after defrosting, but if you don't mind mushy fruit, experiment with raspberries and strawberries, too!
Fridge Stash: When you make dinner (or on the weekend when there's more time), prepare and cook a larger batch of whatever you are making and keep the leftovers in airtight containers.
Of course, try to select food that will hold up well in a bento. If the meal you are making isn't bento-friendly, but its ingredients are, just cut up extra vegetables, etc. and keep them in the fridge, then continue making your meal for the evening.
Stir-fried vegetables, carrots, beans, and capsicum/sweet peppers all work well, as does meat such as beef soboro. (They should last about a week in the fridge).